The present invention relates to a d.c. block capacitor circuit which has particular, but not exclusive, application in integrated circuits, especially integrated direct conversion receivers.
A known d.c. block capacitor circuit comprises an amplifier having an input to which a capacitor is connected, the input being coupled to a reference voltage point by a way of a resistance. In operation, when a low frequency signal is applied to a signal input terminal to which the capacitor is connected, the capacitor behaves as an open circuit and blocks the d.c. so that the gain of the circuit is zero. In contrast, at high frequencies the capacitor acts as a short circuit and the gain will be the normal gain of the amplifier. The disadvantage of this circuit, when integrated as a monolithic device, is that when long time constants are required the capacitor has to have a large value and cannot be conveniently integrated. Two pins therefore have to be reserved for connecting an external component to the monolithic device. When implementing a circuit, particularly an analogue circuit, as a monolithic device the pin count can be a dominating factor and therefore reducing the requirement for external connections is of importance to a circuit designer.